Fans of Laura Ingalls Wilder's semi-autobiographical Little House books have long been familiar with Laura's older sister, Mary, and the explanation that scarlet fever caused her vision loss. But research released Monday says that diagnosis is not very probable.

Instead, Mary's blindness at age 14 was most likely the result of a brain infection, viral meningoencephalitis, says the study in Pediatrics.

Scarlet fever (strep throat with a rash) was one of the most common infectious causes of death in American children between 1840 and 1883, with fatality rates of 15% to 30% of cases, says the study.

But few cases of permanent blindness have been linked to the disease, and why scarlet fever would cause permanent vision loss is unclear, says lead study author Sarah Allexan, a first-year medical student at the University of Colorado School of Medicine. She worked with senior author Beth Tarini, an assistant professor of pediatrics at the University of Michigan. Tarini conceived of the project a decade ago when she was in medical school, intrigued by the scarlet fever/blindness connection that she recalled from the Little House stories.

In the tradition of other modern medical sleuths who diagnosis disorders in prominent historical figures and subjects in art and literature, researchers analyzed both historical and medical evidence, including local news reports citing Mary's illness in 1879, firsthand accounts in Ingalls Wilder's memoirs, school registries, and epidemiologic data on blindness and infectious diseases in the late 19th century.

The researchers say meningoencephalitis, an inflammation of the protective membranes covering the brain and of the brain itself, could explain both the blindness and Mary's other symptoms — fever, headaches, weakness, facial paralysis — described in Ingalls Wilder's 1930 memoir, Pioneer Girl, the basis for the book series, which made its debut in 1932.

"It could also lead to inflammation of the optic nerve that would result in a slow and progressive loss of sight," says Tarini.

"There have been a number of theories about the cause of Mary's illness," including measles and stroke, says Nancy Tystad Koupal, director of the South Dakota State Historical Society Press, which will publish Pioneer Girl later this year.

But scarlet fever's deadly impact was well known when Ingalls Wilder wrote her books, and the malady had a place in other popular books set during the period, such as Frankenstein and Little Women, says Allexan.

'Little House on the Prairie' was based on a family living in the Midwest.(Photo: Handout)

Historian and Ingalls Wilder biographer William Anderson thinks "the use of scarlet fever was a literary device" in By the Shores of Silver Lake, the Little House book that cites scarlet fever as the cause of Mary's blindness. "I don't believe medical science was far enough along in 1879 to put a label on her problem," he adds.

"Much of what she wrote was about surviving, overcoming horrible things," Allexan says. "Perhaps scarlet fever seemed like more of a challenge to overcome."

Anderson says he doesn't believe the new research diminishes in any way the historical value of Ingalls Wilder's books.

"To scholars, it means, yes, she fictionalized slightly, she changed some things around to get the story told," he says. "But the essence of the story she told is true."